29. Determination of carbonate in minerals by gravimetric method
Carbonates are important components of many minerals, the most widespread are calcite CaCO3, dolomite (a
mixture of CaCO3 and MgCO3, and others. These minerals are raw materials for obtaining lime and cements.
Analysis of the content of carbonates in the rock is therefore substantial for many branches of mass
production.
Carbonates are decomposed by diluted acids and gaseous CO2 is produced. There are two gravimetric methods
which apply this reaction: determination of the loss of weight of carbonate due to the escape of carbon
dioxide (indirect method), or the CO2 evolved is absorbed in a suitable medium and the increase in weight of
the absorbent is determined (direct method). The direct method is more precise and will therefore be
described here.
For decomposition of carbonates we will use dilute HCl, HClO4, or H3PO4 acids. The latter seems to be the most
convenient because it is not volatile (why?) and its reaction with the mineral can be easily controlled. When
using dilute HCl one should insert a short water-cooled condenser between the reaction flask and the
absorption train.
We will need two absorbents.
One of these is for water vapor (anhydrous CaCl2 14-20 mesh, anhydrous CaSO4, or anhydrous Mg(ClO4)2).CaCl2
is less useful, because it contains sometimes a little free lime which will absorb carbon dioxide, too. This can be
avoided by saturating the U-tube containing calcium chloride with dry CO2 for several hours and then to
displace the gas by a current of pure dry air before use. On the other hand, Mg(ClO4)2 is expensive.
The second absorber is for carbon dioxide. In general use is soda-lime (a mixture of NaOH with Ca(OH)2,aq,
available also as self-indicating granules, “Carbosorb”. They change color when the absorber is exhausted),
which reacts as follows:
2NaOH + CO2 → Na2CO3 + H2O
Water is a product in this reaction, hence it is important to fill 25-35% of the tube with any of desiccants listed
above.
Fig.1. The absorption U-tube as used in this experiment.
The aim of this exercise is to determine the content of carbonates (calculated as calcium carbonate) in a
sample of mineral. The sample is decomposed evolving stoichiometric amount of CO2 which is next absorbed
in the U-tube as in the Figure 1. The increase in mass of the U-tube after reaction is attributed to the mass of
CO2absorbed in it.
Chemicals and equipment:
- H3PO4, NaOH in granules, anhydrous CaCl2prepared in prior of useas described above;
- Four U-tubes, a 100 mL flask, a dropping funnel, small U-tube for NaOH-guard, a bubbler, a stand (see
Figure 2 below);
- Mortar with pestle.
© Faculty of Chemistry, University of Wrocław, Analytical Chemistry Dept., Gravimetry-carbonates. Task 29 - p. 1
Attention! When operating with NaOH use laboratory gloves and protecting glasses – strong bases are very
harmful!
Procedure:
1. Fill the dropping funnel (B) and the bubbler (D) with phosphoric(V) acid, the guard U-tube (C) with NaOH.
Two U-tubes (E and H) should be filled with CaCl2 (prepared by the laboratory staff in prior to use), and
two others (F and G) as shown in Fig. 1. Fit up the apparatus shown in Figure below, but without U-tubes F
and G. Ask your teacher for details of connections.
Fig.2. The experimental setup.
2. Make sure the apparatus is gas-tight.
3. Grid a piece of the mineral under analysis to powder. Weigh out accurately 0.4-0.7 g of the sample into
the flask A (clean and dry). Wipe the two absorbing U-tubes F and G with a clean linen handkerchief or
cloth and leave them in the balance case for ca. 30 min. Open the taps of these U-tubes momentarily to
the air and weigh separately. Now fit them in the drying train.
4. Open the taps of the U-tubes.
5. Run in phosphoric acid from the tap funnel into the flask. The acid should cover the solid, 25 mL should be
more than suffice. Heat the flask carefully. No more than 2 bubbles of gas per second should pass through
the bubbler D.
6. After 25-35 min heat the flask to boiling for 2-3 min. Remove the heater, disconnect the reaction flask A
from the bubbler, connect the nitrogen line to the bubbler D (to its open end) and draw dry N2 through
the apparatus at the rate of 2-3 bubbles per second, for 15-20 min.
7. Remove the tubes F and G, close their taps, treat as before and weigh them.
8. From the increase in weigh, calculate the percentage of CO2 and of corresponding CaCO3 in the sample.
MCO2=44.01 g/mol; MCaCO3=100.09 g/mol
Report:
The report should contain the masses recorded and the results as percentages as described above.
Sources:
Textbooks, in particular „Vogel’s textbook of quantitative chemical analysis”, G.H. Jeffery, J. Bassett, J. Mendham, R.C.
th
Denney, Longman, Great Britain, 5 edition, 1989 (from which the Figures were copied).
© Faculty of Chemistry, University of Wrocław, Analytical Chemistry Dept., Gravimetry-carbonates. Task 29 - p. 2